1. Field of the Invention
This invention generally relates to a facsimile machine, and, in particular, to a facsimile machine having a function of shifting down its data transmission speed depending on network conditions.
2. Description of the Prior Art
Typically, use is made of a telephone network as a transmission line for facsimile data transmission. In the case of a telephone network, the network condition is liable to change by noises or the like, in particular when a long-distance communication, such as oversea communication, is to be carried out. When a facsimile communication is carried out under such condition, there occurs many errors in transmission of image information so that there may be a case in which an image cannot be reproduced accurately at the receiver.
In a typical facsimile machine, for example, of G3 type, the network condition is examined by MODEM training prior to the initiation of transmission of image information, and the data transmission speed for transmitting image information is selectively determined depending on the status of the network condition.
CCITT has recently announced an additional recommendation for a communication control procedure called an error correction mode or simply referred to as ECM hereinafter in the form of Annex A to Recommendation T.30 on which facsimile machines of the G3 type are based. In the case of carrying out facsimile communication according to this ECM mode, image information is transmitted with a block comprised of a plurality of data frames as a unit, and when errors have occurred in the transmitted image information, those data frames for which the errors have occurred are retransmitted. And, if the number of retransmissions has exceeded a predetermined value, the data transmission speed is shifted down to carry out retransmission of the data frames at a lower data transmission speed.
Suppose that the network condition was excellent immediately after the initiation of transmission and then the network condition became extremely inferior after the initiation of transmission of image information. In this case, the data transmission speed was initially set, for example, at 9,600 bps for transmission of image information. However, the network condition deteriorates and many errors start to occur in transmitted data. As a result, a retransmission process is carried out repetitively over a predetermined number of times. If the data are not transmitted properly during this retransmission process, the data transmission speed is shifted down, for example, to 7,200 bps, and another retransmission process with this shifted down speed is carried out over a predetermined number of times. If the data are still not transmitted properly, the data transmission speed is further shifted down, for example, to 4,800 bps. And, in this manner, the data transmission speed may finally be shifted down to 2,400 bps.
In this manner, according to the conventional ECM mode, since the data transmission speed is shifted down one step at a time when the deteriorated network condition has occurred, it tends to take time until the retransmission of image information has been successfully carried out, which tends to prolong the overall communication time period.
In the above-described ECM mode, the image information after compression by coding is divided into frames of 256 or 64 bytes (octet; 1 byte=1 octet=8 bits) from the beginning thereof and the image information of one frame is shaped into a frame FLM of the type of HDLC (High Level Data Link Control) procedure as shown in FIG. 14a before transmission. The frame FLM includes a (front) flag sequence F of a predetermined bit pattern, an address field A of a predetermined bit pattern (global address), a control field C of a predetermined bit pattern unique to a particular facsimile machine, an information field I, a frame check sequence FCS for error detection, and a (rear) flag F arranged in the order mentioned. The information field I includes a facsimile control field FCF in which a facsimile transmission procedure signal is arranged and a facsimile information field FIF in which various information added to the facsimile transmission procedure signal is arranged. In this case, a facsimile coding data FCD of the facsimile transmission procedure signal is arranged in the facsimile control field FCF, and a frame number FNo indicating the order of the frame and a frame data FDc which is a coding of one frame size FSZ are arranged in the facsimile information field FIF. Since the frame number FNo is defined by eight bits of binary numbers, it can range only from 0 to 255 consecutively, so that a consecutive series of 256 frames is set as a block, and it is so structured that the receiver requests retransmission of such a block. If the image information of one page could not have been transmitted by one block, the remaining portion of the image information is inserted into the next block for transmission.
When the receiver makes a request of retransmission, it sends a frame of facsimile transmission procedure signal PPR (Partial Page Request signal) to the transmitter. It is to be noted that although the facsimile transmission procedure signal is transmitted in a frame format similar to that of this partial page request signal PPR with necessary parameters, it will be simply referred to as a PPR signal in the following description for the sake of convenience. This PPR signal contains a particular bit pattern PPR, which indicates it to be a PPR signal, in the facsimile control field FCF and also contains an error map data EMp of 256 bits in the facsimile information field FIF. In this error map data EMp, among one block of block data which has been transmitted, a data "0" is assigned to each of the frames for which no transmission error has occurred and a data "1" is assigned to each of the frames for which one or more errors have occurred, and these assigned data are arranged in the order of the frames. Upon receipt of this PPR signal, the transmitter retransmits only the data of those frames for which data "1" has been assigned in the error map data EMp to the receiver. By repetitively issuing this request for retransmission until all of the data errors have been eliminated, a received image without errors can be recorded at the receiver.
In transmitting image information as described above, the frame size may be set at one of two kinds, i.e., 256 bytes and 64 bytes. When this frame size has once been set during a pre-transmission procedure between the transmitter and the receiver, it remains valid until the transmission of one page of image information has been completed and it cannot be altered. When this frame size is set in a facsimile machine, if it is set semi-permanently, for example, by operating a switch or the like mounted on an internal circuit board provided in the facsimile machine, the following disadvantages could occur.
That is, if the frame size semi-permanently set at 256 bytes, since the number of frames constituting image information may be fewer, the amount of information to be added to the frame data may be less. Thus, as compared with the case in which the frame size is 64 bytes, the transmission time period may be shortened. However, for example, as shown in FIGS. 15a and 15b, if burst errors BN1 and BN2 have occurred in a transmission line, the amount of data to be retransmitted increases significantly as compared with the case in which the frame size is 64 bytes. Accordingly, if the network condition is inferior, the retransmission time period could be longer for frame size (FSZ1) of 256 bytes than for frame size (FSZ2) of 64 bytes, so that the overall image information transmission time period could also be longer for the 256 byte case. In this manner, in the case where transmission is to be carried out in the ECM mode, the frame size for which transmission can be carried out efficiently differs depending on the network condition. On the other hand, if the network condition is extremely good, since there is produced virtually no data errors in image transmission, there is no need to use the ECM mode. Similarly, if the network condition is extremely bad, since many data errors would occur also during retransmission, the use of the ECM is meaningless because it only prolongs the transmission time period without merit.
In a prior art facsimile machine having both of ECM and normal modes of operation, since the selection between the ECM and normal modes was not always made properly and the frame size for use in the ECM mode was not always set properly, the ECM mode was not used effectively at all times. It may be so structured that such setting be carried out by an operator; however, in such a case, since the operator is required to determine appropriate parameters before transmission, which is cumbersome to the operator and also impractical.
In the conventional G3 type normal mode, when transmitting image information in facsimile communication, the image information is first coded and the thus coded image information is normally stored in a FIFO buffer temporarily. And then the coded image information is read out of the buffer and transmitted to the receiver at the speed corresponding to the data transmission speed set in the MODEM. In this case, the FIFO buffer serves as an interface so as to absorb the discrepancy in timing between the coding of image information and transmission of image information from the MODEM.
In the case of the ECM mode, when constructing a HDLC data frame, the insertion of "0" data so as to avoid the occurrence of the same bit pattern as that of the flag in frame data and the CRC calculation for error checking operation are carried out. For this reason, in the case of facsimile communication in the ECM mode, during transmission, a block of the coded image information is temporarily stored in the buffer memory to thereby facilitate the carrying out of such processes as insertion of data "0" in the frame data and CRC calculation and to allow for the retransmission of image information. Thus, both in the conventional G3 type normal mode communication and the ECM mode communication, the image information to be transmitted is stored in a buffer. And, in the prior art facsimile machines, two separate buffers for storing image information were provided for the above-described two kinds of communication.
As described above, in accordance with the conventional ECM mode, when the network condition deteriorates after the initiation of transmission of image information, the communication time period tends to become prolonged. Furthermore, in the prior art facsimile machines, the ECM mode was neither properly set nor effectively used. Moreover, in the prior art facsimile machines, two separate buffers for temporarily storing image information were provided, which constituted one of the reasons for increased cost.
Japanese Patent Laid-open Pub. No. 58-198964, published Nov. 19, 1983, discloses a data transmission system using a shift-down/shift-up condition determining circuit 7 which calculates the number of retransmission operations selectively carries out either a two-step shift-down operation or a one-step shift-down operation depending on the manner of occurrence of retransmission operations, i.e., either continuous or continual. Japanese Patent Post-examination Pub. No. 60-36145, published Aug. 19, 1985, discloses a signal transmission system in which retransmission is carried out only when transmission errors have occurred in frame information having a high degree of importance and retransmission is not carried out for frame information having a low degree of importance even if transmission errors have occurred so as to shorten the overall transmission time period.